feat(dashboard): unified Hosts widget + grouped widget picker

Bring the dashboard in line with the unified host IA + improved displays.

- New core "Hosts — Overview" widget (/hosts/overview/widget): one row per host
  combining monitor status (ping dot + latency, uptime 24h) with the agent
  glance (CPU / memory / disk root + stale flag), each row linking to the host
  hub. Reads agent data from the generic PluginMetric table via a core-safe
  _agent_overview_by_host helper (no host_agent import); freshness vs the
  plugin's stale window. The granular Ping/DNS/Uptime/Agent widgets stay.
- Group the add-widget picker into Core monitors / Monitoring capabilities /
  Integrations (a `group` field on every WIDGET_REGISTRY entry + section
  headings in _edit_panels.html), matching the Settings → Plugins taxonomy.
- Fix the agent fleet widget rows to link to the host hub (/hosts/<id>) instead
  of the old /plugins/host_agent/<id>/ page.

Scribe #903.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

steward/templates/dashboard/_edit_panels.html

@@ -37,8 +37,13 @@
   <div>
     <div class="section-title" style="margin-bottom:0.75rem;">Available Widgets</div>
     {% if addable %}
+    {% set _groups = [("core", "Core monitors"), ("capability", "Monitoring capabilities"), ("integration", "Integrations")] %}
+    {% for _gkey, _glabel in _groups %}
+    {% set _gw = addable | selectattr("group", "equalto", _gkey) | list %}
+    {% if _gw %}
+    <div style="font-size:0.72rem;text-transform:uppercase;letter-spacing:0.04em;color:var(--text-dim);margin:0.85rem 0 0.35rem;">{{ _glabel }}</div>
     <div style="display:grid;gap:0.5rem;">
-      {% for w in addable %}
+      {% for w in _gw %}
       <div class="card" style="margin-bottom:0;padding:0;overflow:hidden;">
         <details>
           <summary style="display:flex;align-items:center;justify-content:space-between;gap:0.75rem;
@@ -97,6 +102,8 @@
       </div>
       {% endfor %}
     </div>
+    {% endif %}
+    {% endfor %}
     {% else %}
     <div style="color:var(--text-muted);font-size:0.85rem;padding:1rem 0;">
       No plugin widgets available. Enable plugins in Settings → Plugins.

steward/templates/hosts/overview_widget.html

@@ -0,0 +1,42 @@
+{# Unified Hosts widget — monitor status + agent glance per host, links to the hub. #}
+{% if hosts %}
+<div style="display:grid;gap:0.1rem;">
+  {% for host in hosts %}
+  {% set ping = latest_pings.get(host.id) %}
+  {% set dns = latest_dns.get(host.id) %}
+  {% set ut = uptime.get(host.id, {}) %}
+  {% set a = agent.get(host.name, {}) %}
+  {% set down = host.ping_enabled and ping and ping.status.value != 'up' %}
+  <div style="display:flex;align-items:center;gap:0.6rem;font-size:0.82rem;padding:0.3rem 0;border-bottom:1px solid var(--border);">
+    <span class="dot {% if down %}dot-down{% elif host.ping_enabled and ping %}dot-up{% else %}dot-dim{% endif %}"
+          title="{% if not host.ping_enabled %}ping off{% elif ping %}{{ ping.status.value }}{% else %}no data{% endif %}"></span>
+    <a href="/hosts/{{ host.id }}"
+       style="flex:1;min-width:0;white-space:nowrap;overflow:hidden;text-overflow:ellipsis;font-weight:500;">
+      {{ host.name }}
+    </a>
+    <span style="display:flex;gap:0.75rem;font-variant-numeric:tabular-nums;color:var(--text-muted);font-size:0.75rem;flex-shrink:0;">
+      {# Monitors #}
+      {% if host.ping_enabled and ping and ping.response_time_ms is not none %}
+      <span title="Ping latency"><span style="color:var(--text-dim);">ping</span> {{ "%.0f"|format(ping.response_time_ms) }}ms</span>
+      {% endif %}
+      {% if ut.get('24h') is not none %}
+      <span title="Uptime 24h"><span style="color:var(--text-dim);">24h</span> {{ "%.1f"|format(ut['24h']) }}%</span>
+      {% endif %}
+      {# Agent glance #}
+      {% if a %}
+      {% if a.get('cpu_pct') is not none %}<span title="CPU"><span style="color:var(--text-dim);">cpu</span> {{ "%.0f"|format(a.cpu_pct) }}%</span>{% endif %}
+      {% if a.get('mem_used_pct') is not none %}<span title="Memory"><span style="color:var(--text-dim);">mem</span> {{ "%.0f"|format(a.mem_used_pct) }}%</span>{% endif %}
+      {% if a.get('disk_root') is not none %}<span title="Root filesystem (/)"><span style="color:var(--text-dim);">disk /</span> {{ "%.0f"|format(a.disk_root) }}%</span>{% endif %}
+      {% if not a.fresh %}<span title="Agent data is stale" style="color:var(--yellow);">stale</span>{% endif %}
+      {% else %}
+      <span style="color:var(--text-dim);" title="No agent reporting">no agent</span>
+      {% endif %}
+    </span>
+  </div>
+  {% endfor %}
+</div>
+{% else %}
+<div class="empty" style="color:var(--text-muted);font-size:0.85rem;padding:0.5rem 0;">
+  No hosts yet. <a href="/hosts/new">Add one.</a>
+</div>
+{% endif %}